Cordierite ceramics are widely used as a high temperature gas filter or a catalyst carrier for carrying an exhaust gas purification catalyst of an automobile engine because cordierite ceramics show low thermal expansion and have excellent thermal resistance and thermal shock resistance. For example, cordierite ceramics are adapted as the main component of a honeycomb shaped porous catalyst carrier for automobile exhaust gas purification and required to obtain excellent thermal resistance and thermal shock resistance.
Cordierite ceramics used as automobile engine exhaust gas purification catalyst carriers are widely used as honeycomb shaped ceramics. A cordierite honeycomb structure used as the automobile engine exhaust gas purification catalyst carrier is required to have low thermal expansion in the direction in parallel with the faces of the partition walls of a honeycomb shape in comparison with the other directions.
For example, it has been known that thermal expansion in the direction in parallel with the faces of the partition walls of the cordierite honeycomb structure can be lowered by orienting the c-axis of a cordierite crystal toward the direction in parallel with the faces of the partition walls (see, e.g., JP-A-50-075611). With regard to a method for orienting the c axis of a cordierite crystal with respect to a certain face, many techniques have been disclosed regarding conditions of materials constituting the cordierite-forming raw material, conditions upon forming the raw material for forming cordierite, conditions upon firing (heating) a formed article of the raw material for forming cordierite, and the like (see, e.g., JP-A-50-075611, JP-A-53-082822, and JP-A-64-003067).
Examples of the material constituting a cordierite-forming raw material include talc, kaolinite, silica, and alumina. It has been noted that, of these, alumina inhibits reduction in thermal expansion coefficient of the resultant cordierite ceramics (see, e.g., JP-A-50-075611).
For example, when the particle diameter of α-alumina used as a material for the raw material for forming cordierite is 3 μm or less, a reaction with talc in firing proceeds at 1300° C. or less. The reaction between α-alumina and talc inhibits a reaction between talc and kaolinite important for generation of cordierite having low expansion (see, e.g., JP-A-61-256965). On the other and, when the particle diameter of α-alumina used as a material for the raw material for forming cordierite is 15 μm or more, it is necessary to raise reaction temperature for cordierite generation, and the resultant cordierite ceramics have high thermal expansion coefficient with poor thermal resistance and thermal shock resistance.
As described above, since alumina used as a material for a cordierite-forming raw material affects thermal resistance and thermal shock resistance of the cordierite ceramics as final products, there have been proposed methods where action inhibiting thermal resistance and thermal shock resistance by alumina is reduced. However, there has been disclosed neither constitutions nor conditions for making alumina used as a material for a cordierite-forming raw material positively function to improve thermal resistance and thermal shock resistance of cordierite ceramics.